To make the zabaglione, put the egg yolks, sugar and vanilla seeds in a round-bottomed steel or Pyrex bowl (not aluminium or it may go a nice grey colour).
Beat well with a whisk for 2-3 minutes then beat in the Marsala. Hold the bowl with a cloth and fit it over a pan containing 3-4cm of simmering water. Whisk continuously for a few minutes (use an electric whisk if you prefer) until the mixture is thick and frothy.
To serve, put the strawberries in bowls or plates and pour over the warm zabaglione.